A conventional tuning fork type quartz crystal resonator has mainly two vibration modes. They are fundamental vibration and overtone having the resonance frequency six times as much as the frequency of fundamental vibration. These two vibrations have similar high quality factors (several tens of thousand.about.a hundred and tens of thousand). So this quartz crystal resonator has low crystal impedance. In the case where such a tuning fork type quartz crystal resonator is added in an oscillator circuit, overtone is produced in spite of desiring fundamental vibration due to value of circuit elements, variation of temperature characteristics thereof, supply voltage and dispersion of resonators.